A heart attack
is caused by a prolonged reduction in blood/oxygen
supply to the heart as a result of a blockage in one of the vessels which supplies the heart. Tissue/cell death then occurs as a result of this and the area is said to be infarcted.
A cardiac arrest
simply means that the heart has stopped pumping blood around the body due to a disturbance in heart rhythm,- most commonly ventricular fibrillation
. Many different things can cause this. A heart attack itself can lead to a cardiac arrest if blood supply is not restored to the heart quick enough and/or blood supply is completely blocked off.
Unconfirmed reports are suggesting MJ died of an overdose of prescription drugs which can cause both respiratory and cardiac arrest.
You can have a heart attack without experiencing the changes in rhythms that can lead to a cardiac arrest. For example, blood supply may only be partially cut off. If you have a cardiac arrest, you lose consciousness almost instantly.